Fix breaking change in cortex-m 0.5.11.

Fix it by sticking with cortex-m 0.5.6.

Author: wpd

src/11-usart/auxiliary/Cargo.toml

@@ -5,7 +5,7 @@ name = "aux11"
 version = "0.1.0"
 
 [dependencies]
-cortex-m = "0.5.6"
+cortex-m = "=0.5.6" # 0.5.11 introduces a breaking change.  Use 0.5.6, since we know it works for this example
 cortex-m-rt = "0.6.3"
 panic-itm = "0.4.0"